1. Field of the Invention
The invention relates to paper manufacturing machines and in particular to a nozzle chamber of a multilayer headbox of such a machine.
2. Description of Related Technology
A multilayer headbox of a paper manufacturing machine utilized for the introduction of at least two fiber suspension streams to a forming section of a paper manufacturing machine, each suspension stream spanning across a width of the machine, is disclosed in DE 43 29 810. Such a machine includes a nozzle chamber having a machine-wide outlet gap at a downstream portion thereof with respect to a direction of fluid flow through the machine. The nozzle chamber is defined by two machine-wide stream guide walls and two side walls. The guide walls terminate in the vicinity of the outlet gap of the nozzle chamber.
The headbox disclosed in DE 43 29 810 also includes at least one machine-wide lamella having a body portion and a tip made from a hard material. The lamella is disposed in the nozzle chamber and separates at least two fiber suspension streams flowing through the headbox all the way to the outlet gap. The function of the lamella is to keep the two fiber suspension streams separated so that no mixing of the different streams occurs until the streams are ultimately combined into a single stream in as undisturbed form as possible. Thus, no disturbances should occur in the stream. In this connection, the design of the end of the lamella must be taken into consideration, because the area about the lamella end is frequently the origin of flow disturbances. Various attempts have been made to avoid such disturbance in the fluid flow. Among others, according to DE 43 23 050 A1, it has been proposed to design the lamella in such a way as to cause an increased convergence in the flow channel, so that, as a result of an increase in the flow velocity in the vicinity of the lamella end, there results a reduction of disturbances due to frictional turbulence.
Regarding the constructional design of the end of the lamella, until now the opinion prevailed that the end of the lamella should not be made very sharp for reasons of manufacturing technology, costs and operational safety. However, a blunt lamella end may produce flow turbulence or periodic separations of turbulent flow, and as a consequence, vibrations are transferred to the end of the lamella or to the entire lamella. As a result, disturbances occur in the formation of the paper web and/or local undesirable mixing of neighboring pulp streams may occur. Consequently, in the finished multilayer paper, the layers will no longer be clearly distinguished from one another at some points, for example, with regard to different colors.
An attempt to avoid these disadvantages is disclosed in DE 43 29 810 which teaches providing slits or grooves in the essentially blunt lamella end or applying a thin foil which protrudes beyond the lamella end. However, such measures have been found to not be fully satisfactory.